Chapter 1. INTRODUCTION

Reference to a covered bridge often sparks an image of a quaint
setting with a narrow, but inviting, timber tunnel crossing of a
stream. For some, the image is of a more substantial structure
crossing a raging river, withstanding the rigors of time and
nature. At one time, the United States reportedly had 14,000 of
these unique bridges dotting the countryside over a surprisingly
large area. Now, fewer than 900 of the historic structures survive,
and pressures grow to help preserve them from replacement,
abandonment, vandalism, and arson. Federal funding over the past
decade has grown to augment that of the States, owners, and others
to find comprehensive and proven means of maintaining the ability
of these vestiges of our bridge-building heritage to continue to
serve current and future generations.

This manual is intended to provide comprehensive support to
those involved with maintaining, assessing, strengthening, or
rehabilitating covered bridges, especially heavy timber truss
bridges. Although there are covered bridges throughout the world,
this manual focuses on covered bridges within the United States.
There is brief reference to those in other countries, primarily to
provide a historical context of the development of timber truss
bridges.

According to the World Guide to Covered Bridges (World
Guide), of the approximately 1600 covered bridges in the
world, roughly 880 are in the United States.[1] Even
though there are many similarities among these bridges, no two were
absolutely alike the day they were opened to traffic, and none have
undergone identical use and maintenance to date. Therefore, even by
limiting coverage to North American bridges, a fairly large and
varied population is addressed.

This manual focuses on those covered bridges supported (or at
least supported at one time) by longitudinal trusses built of
relatively large (heavy) timber components. This manual deals only
with covered bridges, and not with those bridges that are covered.
The former describes a structure that earns its keep—one that is as
it appears to be—an authentic covered bridge. The other, so-called
covered bridges (usually girder-supported bridges with some sort of
shed on top) just happen to support a roof and walls, and are not
generally considered legitimate covered bridge structures.

Figure 3 shows a classic historic covered bridge—the Taftsville
Bridge in Woodstock, VT. This two-span bridge, supported by unique
trusses with heavy arches, was built in 1836, making it one of the
older covered bridges in the United States.

Some authentic covered bridges have been retrofitted to remove
their timber floor system and replace it with an independent
system. The remaining timber-framed covering must still support its
own weight, along with wind forces, and potentially snow. This
manual does not separate these retrofitted structures from other
authentic complete bridges; —the discussion can deal with either,
and will be useful in establishing ways to evaluate the timber
trusses and means of maintaining and/or rehabilitating them.

Information is culled from both readily and not-so-readily
available references. Because little has been written about the
technical aspects of covered bridges, information is provided based
on both the experience and expertise of the authors. It is worth
noting that some of the information in this manual represents
opinions regarding best practices, even though these practices may
not be commonly or widely accepted.

The content of this manual is strongly influenced by the
Principal Investigator’s involvement in the comprehensive statewide
study of 75 covered bridges in Vermont. The work, was performed by
a consulting firm under contract to the Vermont Agency of
Transportation, and was concluded in 1995. The study involved
preparing a long-term preservation plan for each bridge based on
the condition of the bridge and the traffic needs at the site. The
team responsible for this manual has had firsthand hand engineering
involvement on more than 110 bridges in many States. The population
of covered bridges in the United States is diverse, in large part
due to the nature of their construction. Few builders traveled long
distances and features favored by one builder were different from
those in another area. Hence, this manual presents a comprehensive
discussion of the multiple facets of covered bridges, but cannot
include all aspects.

While many of the references are useful in covering several
topics, they will be introduced with each topic. Some very general
and useful references are cited throughout the manual. The most
pervasive background sources, specific to their specialized topics,
are:

For bridge specifications—Standard Specifications for
Highway Bridges, as adopted by the American Association of
State Highway and Transportation Officials (AASHTO).[2]
These specifications were published first in 1931 and have been
modified and expanded regularly. The latest edition of these
specifications is the 17th edition of 2002. Chapter 9 of
this Covered Bridge Manual clarifies some of the confusion
regarding the differences between the AASHTO “Standard
Specifications” versus the “Load and Resistance Factor
Specifications” as they relate to covered bridge practice.

For timber specifications—National Design Specifications for
Wood Construction (NDS®) and its Supplement:
Design Values for Wood Construction, published by the American
Forest and Paper Association.[3] These specifications
were published first in 1944 and have been modified and reissued
regularly ever since. The most recent edition was published in
2001. The NDS, is commonly cited and is the basis of timber-related
provisions published by other organizations (including the AASHTO
bridge specifications), is commonly cited.

For an excellent overall reference regarding timber bridges
is—Timber Bridges Design, Construction, Inspection, and
Maintenance, published by the U.S. Department of Agriculture,
U.S. Forest Service.[4] The latest reprinting is
1990.

For a comprehensive listing of covered bridges, with a synopsis
of relevant information concerning them, see the World
Guide, published by the National Society for the Preservation
of Covered Bridges, Inc.[1] First published in 1956, the
latest edition is 1989. A new edition wais under development as of
2004.

The manual is primarily intended for engineers and historic
bridge preservationists to provide technical and historical
information on preservation of covered bridges. However, it will
also be of interest to others involved with covered bridges,
including the lay persons, owners who may have some knowledge on
the topic, or contractors involved with covered bridges.
Accordingly, the presentation style is somewhat modified, depending
on the topic and perceived audience.

The manual is separated into several sections with a number of
chapters devoted to the specifics of each. The first three chapters
provide background information. The typical covered bridge is
described, along with its setting and terminology. Some general
facts and statistics are provided. A brief historical development
is included to help explain how timber bridges evolved and spread
across the United States.

The description of bridge components section provides
descriptions of the various truss configurations, discussions of
the floor systems of the bridges, the various ancillary features
that supplement the primary features of trusses and floor, and a
discussion of the foundations that support the bridge.

Technical engineering issues begins with a short chapter that
explains some of the unusual challenges of work with covered
bridges and why their engineering is different from that for other
bridges. Other chapters are devoted to guiding specifications;
clarifications of topics related to wood; loads and the increased
importance of the weight of the structure and the special handling
of snow in combination with vehicular traffic; nuances related to
forces, stress analysis and design; and a lengthy discussion of
connections.

The existing bridges section is the heart of this manual. The
first chapter of this section is devoted to evaluating a bridge.
The next chapter provides a discussion of repair and strengthening
of bridges, along with examples of three recent projects. The
following chapters include a guide for preservation actions and a
summary of historic considerations related to this type of work. A
comprehensive state-of-the-art guide for preservative treatment of
wood in covered bridges is presented as the last technical
chapter.

The final chapter provides a list of references and other
sources of information.

The appendices include case studies that provide more indepth
examples of repair and strengthening of extant bridges. Examples of
recent authentic-type construction at new sites and replica bridges
intended to generally duplicate a destroyed bridge, are also
included as appendices in this manual. The growing number of new
covered bridges warrants inclusion in this manual, because many of
the same issues are relevant to them as for historic bridges are
also relevant to new bridges.